Australia's Artesian Basin - $14 Billion down the drain each year

Because it is a closed system, the removal of water is accompanied by a reduction in the water space in the fissure openings in the rock. The fissures close. The water pressure is reduced, the pressure on the rocks is less, and the rocks expand into the fissure openings. There is a reduction in discharge capacity of the aquifer.

A characteristic feature of these rocks is their plasticity and capacity to flow under shear stresses. At depth, these rocks do not normally have open joints or fissures, as the rock itself simply flows or changes shape to close the openings. The rocks may have fractures, but not open joints. Open joints and fissures may be evident in near surface rocks, but not normally at depth.

In the Great Artesian Basin there are fissures in the sandstones in the aquifer. These open fissures were originally formed as closed fractures during crustal movements, and then opened and sustained by the high water pressure in the aquifer. Over geological time there was a hydraulic jacking effect, which effectively opened up joints in the strata over vast distances.

The aquifer was made possible by these high water pressures.

The effect of a reduction in aquifer water pressure is quite catastrophic.

The open fissures close as the rocks respond to the effect of the lower water pressures and the driving force of the weight of the overlying sediments. There is a loss in potential energy as the overlying rock mass subsides.

It is definite and slow change. For all practical purposes it is quite irreversible, as the matter can only be corrected by applying water injection pressures sufficient to carry much of the weight of the overburden over a long period of time.

In effect, the depressurizing of the aquifer has resulted in a huge loss in potential energy, and that energy, and more, has to be re-applied to reverse the process.

The pressure drop since the first development of the Great Artesian Basin is assessed at up to 100 metres of water pressure head.

It is inevitable that the closure of fissures arising from that loss of pressure is still occurring, and that we may expect continuing closure for some time. It is much worse if boreholes are permitted to flow freely, as that accelerates the process. The immediate capping of all open bores is absolutely vital if we are to have any prospect of retaining the Great Artesian Basin at anywhere near present levels of useful output.

Even within a capped bore there is still the real possibility of leakage from a high-pressure aquifer into the various strata above. Many bores may have to be reconstructed to ensure that there is no internal leakage.

It should be recognised that the dry bores represent the end phase in the permanent destruction of the aquifer. There is little prospect of long term supply by using pumping to lift water from a dry bore, as the lowering of the water pressure at the pump intake would lead to final closure of the fissures.